[mikegerwitz]

In addition to what the others said, it could be used to correlate you across multiple e.g. public wifi hotspots. Imagine some dissident in a repressive regime leaking information and law enforcement checking the logs of the routers for various public places. If they find a MAC at all locations, they might be able to check security cameras and see what individuals were present at that time and correlate that MAC with an individual, and then further use that information to track their movements.

Yes it can be spoofed and someone could potentially be framed, but it's just more information that can be used in conjunction with other data to help deanonymize a person.